The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Plasma arc torches, also known as electric arc torches, are commonly used for cutting, marking, gouging, and welding metal workpieces by directing a high energy plasma stream consisting of ionized gas particles toward the workpiece. In a typical plasma arc torch, the gas to be ionized is supplied to a distal end of the torch and flows past an electrode before exiting through an orifice in the tip, or nozzle, of the plasma arc torch. The electrode has a relatively negative potential and operates as a cathode. Conversely, the torch tip constitutes a relatively positive potential and operates as an anode during piloting. Further, the electrode is in a spaced relationship with the tip, thereby creating a gap, at the distal end of the torch. In operation, a pilot arc is created in the gap between the electrode and the tip, often referred to as the plasma arc chamber, wherein the pilot arc heats and ionizes the gas. The ionized gas is blown out of the torch and appears as a plasma stream that extends distally off the tip. As the distal end of the torch is moved to a position close to the workpiece, the arc jumps or transfers from the torch tip to the workpiece with the aid of a switching circuit activated by the power supply. Accordingly, the workpiece serves as the anode, and the plasma arc torch is operated in a “transferred arc” mode.
The plasma arc torch is generally maintained at a predetermined torch height during operation for optimum cut quality. For manual operation, a welder can adjust the torch height based on the arc voltage between the torch tip and the workpiece even for cutting a workpiece with varied height or thickness. For automated plasma arc torches controlled by robots, however, the torch height control becomes difficult or slow based on the arc voltage. Therefore, the robot-controlled plasma arc torches are generally pre-programmed with a fixed torch height. As such, the robot-controlled plasma arc torches can be used to cut limited types of workpieces, such as small workpieces which are more dimensionally stable during cutting or thicker workpieces which are less sensitive to height variations of the workpiece. The robot-controlled plasma arc troches are not suitable for cutting workpieces with height variations.
Moreover, using robotic-controlled plasma arc torches to cut large thin materials poses another challenge in that the large thin materials may have a dimensional tolerance that is greater than the torch height (in the range of 0.020 to 0.150 inches). In other words, the height variations in the workpiece may be greater than the torch height. The large thin materials may have deformed due to heat or residual stress generated during prior operations such as stamping. Without the ability to adjust the torch height during operation, the automated plasma arc torches would not achieve optimum cut quality or may be damaged by the workpiece.